Traditionally the manufacturing of DSA is based on a substrate provided with a layer of catalyst put upon it, like U.S. Pat. No. 3,929,608 where an electrode is described comprising of an electro-conductive metal core of titanium or a titanium alloy and a catalytic coating on the titanium metal surface, where the coating includes at least one substance from the group consisting of the platinum group metals and their oxides. Another example is U.S. Pat. No. 5,503,663, where a stable catalytic coating solution is prepared using soluble compounds of at least two platinum group metals or at least one platinum group metal and at least one soluble compound of a valve metal. Valve metals are frequently described as metals or alloys with the property that they easily form a passivating oxide film which protects an underlying metal from corrosion, as it is also described in U.S. Pat. No. 4,797,182. The metals could include for example titanium, tantalum, niobium, zirconium, hafnium, vanadium, molybdenum, and tungsten, as in U.S. Pat. No. 4,469,581.
Another example is U.S. Pat. No. 3,616,445 describing a titanium or tantalum base electrode having a protective and electro-catalytic layer applied to the faces exposed to the electrolyte, said protective and electrocatalytic layer consisting of mixtures of solid solutions of valve metal oxides and noble metals. The oxide covers oxides of titanium and tantalum whether in the form of TiO2 and Ta2O5 or TiOCl and TaO2C1, or other oxides of these metals.
The patents teach that it has been found to be desirable to have mixed oxides in the catalytic coating in order to provide an anode having a longer lifetime. The main causes of failure of such electrodes is attributed to loss of the active coating by dissolution, or is due to passivation by the formation of a highly resistive TiO2 or Ta2O5 layer between the substrate and the oxide coating, so as to require that the anode be operated at increased potential. However, when they appear as mixed oxides they have a good conductivity.
One known potential solution is to establish a layer of titanium/tantalum oxide at the titanium surface, having a better conductivity than titanium oxide, and with a sufficient stability to prevent the formation of further titanium oxide. This has been described in a number of documents, like U.S. Pat. No. 4,469,581 describing an electrolytic electrode having high durability for use in electrolysis where the generation of oxygen occurs, comprising an electrode substrate of titanium or a titanium-based alloy, an electrode coating of a metal oxide; and an intermediate layer comprising an electrically conductive oxide of tantalum provided between the electrode substrates. Ta2O5 has been confirmed to be suitable as substance forming the intermediate layer.
A number of ways to precipitate the materials onto the conductive object is mentioned in for example U.S. Pat. No. 3,632,498, comprising electrolysis or vacuum-sputtering.
In U.S. Pat. No. 5,314,601 a titanium substrate metal is provided with a highly desirable rough surface characteristic for subsequent coating application. This can be achieved by various operations including etching and melt spray application of metal or ceramic oxide to ensure a roughened surface morphology. Usually in subsequent operations, a barrier layer is provided on the surface of enhanced morphology. This may be achieved by operations including heating, as well as including thermal decomposition of a layer precursor. Subsequent coatings provide enhanced lifetime even in the most rugged commercial environments.
Additionally a layer of pure tantalum could subsequently be placed on the surface and heat treated object, so that the surface layer of tantalum diffuse into the substrate, where another oxidation treatment to oxidize the titanium/tantalum alloy takes place. This is for example described in WO 00/60141 where a tri-layer anode is described with an improved service life when used, where the anode is comprised of a titanium substrate which is roughened and heat treated and subsequently coated with a first coating of tantalum oxide. After the anode is heat treated, it is next coated, preferably by an electrodeposition process with a second coating of platinum. Finally, the anode is coated with a third coating of iridium oxide/tantalum oxide and subsequently heat treated.
A number of processes are used to precipitate the tantalum, such as electrolyse, cladding and CVD. When precipitating tantalum by a CVD, it usually is according to the reaction also described in U.S. Pat. No. 4,294,871:2TACl5+5H2→2TA+10HClSuch processes are known to be controlled in the separate phases of titanium metal and tantalum metal. Additionally any pollutions (like oils, remains of process chemicals, absorbing layer of oxygen, carbon etc.) in the interface between the two pure metal phases will influence the reaction, so in an industrial process it can be difficult to control the formation of the mixed metal. It is needed to carefully control the level of contamination at the surfaces, and to adapt the thickness of the tantalum layer and the method of heat treatment, to get a satisfactory result, especially in relation to the composition of the titanium/tantalum oxide layer. In practice, because of variations in the roughness of the substrate and the tantalum metal, an irregular thickness of the tantalum layer will be obtained, possibly because of dendrites formed by the metal precipitation or general irregularities in the substrate. Because of the irregular layer thickness it is not possible by the following heat treatment to obtain a complete uniform diffusion between titanium and tantalum. The composition of titanium and tantalum in the surface therefore changes from one area to another on the electrode surface, either on micro or macro level. These areas are characteristic for varying conductivity. In case of operation of the electrode, this irregularity means that the current varies across the surface (microcells are formed), and an increased risk of a local breakdown exists, in the same way it is known for non-tantalum containing electrodes.